In general, radiosurgery and radiotherapy treatments consist of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (head, body, etc.) is constructed to determine the exact coordinates of the target within the anatomical structure, namely, to locate the tumor or abnormality within the body and define its exact shape and size. Second, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon finds acceptable, taking into account a variety of medical constraints. During this phase, a team of specialists develop a treatment plan using special computer software to optimally irradiate the tumor and minimize dose to the surrounding normal tissue by designing beams of radiation to converge on the target area from different angles and planes. The third phase is where the radiation treatment plan is executed. During this phase, the radiation dose is delivered to the patient according to the prescribed treatment plan using radiation treatment techniques, such as intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), for example. These techniques are typically used with a radiotherapy system, such as a linear accelerator (linac), equipped with a multileaf collimator (MLC) to treat pathological anatomies (tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering prescribed doses of radiation (X-rays, gamma rays, electrons, protons, and/or ions) to the pathological anatomy while minimizing radiation exposure to the surrounding tissue and critical anatomical structures.
There are many factors that can contribute to differences between the prescribed radiation dose distribution and the actual dose delivered (i.e., the actual dose delivered to the target during the radiation treatment). One such factor is uncertainty in the patient's position in the radiation therapy system. Other factors involve uncertainty that is introduced by changes that can occur during the course of the patient's treatment. Such changes can include random errors, such as small differences in a patient's setup position. Other sources are attributable to physiological changes that might occur if a patient's tumor regresses or if the patient loses weight during therapy. Another category of uncertainty includes motion. Motion can potentially overlap with either of the categories as some motion might be more random and unpredictable, whereas other motion can be more regular. Many other sources of uncertainties exist, such as, missing bolus or fixation device (human error), wrong patient, mechanical failure/calibration error/changes is radiation output, corrupted data (plan is not consistent with calculated dose), wrong delivery machine (patient may be treated on another delivery machine in case the original is not functional at the moment, for example. These uncertainties can affect the quality of a patient's treatment and the actual radiation dose delivered to the target.
The accuracy in delivering a predicted radiation dose to a target based on a predetermined treatment plan, therefore, plays an important role in the ultimate success or failure of the radiation treatment. Inaccurate dose delivery can result in either insufficient radiation for cure, or excessive radiation to nearby healthy tissue and organs at risk (OARs). A radiation dose that is too high may cause serious damage to healthy tissues surrounding the tumor as well as organs located nearby, whereas a dose that is too low may jeopardize the probability of cure. Therefore, a relatively small error in the delivered radiation dose may seriously harm the patient. Quality assurance tools and protocols are therefore needed to verify that the prescribed radiation dose is delivered to the target without jeopardizing the organs at risk and the healthy tissue.
Because of the high complexity and uniqueness of treatment plans, patient-specific pre-treatment (i.e., without the patient in the beam) verification is generally considered a necessary prerequisite to patient treatment. Pre-treatment verification includes procedures to compare the whole or at least part of the intended treatment plan with measurements of corresponding radiation beams delivered by the linear accelerator (linac) outside the patient treatment time.
Dosimetric verification is one of the pre-treatment protocols implemented for radiation therapy treatments. Dosimetric verification includes verification that the dose distribution delivered is in fact the dose distribution predicted to be delivered to the patient. Because of the increased beam delivery complexity offered by some of the radiation therapies, such as (IMRT) and (VMAT) treatments, dosimetric verification for treatments require rigorous verification of the radiation dose delivery.
In established dose verification methods, integrated dose distribution images are compared against dose images predicted by the treatment planning system (TPS) using a gamma evaluation method. The gamma evaluation method is widely used in dose measurements, because it combines spatial errors and dose level errors in a single value. The weakness of such an evaluation, however, is that all measurements points are evaluated based on the same criteria, even though the evaluation criteria may be too loose or too rigorous for certain points. A loose evaluation criteria may validate dose delivery even though the detected dose discrepancies may be too high for an organ at risk (overdose or hotspot generated in an organ at risk is much more severe than an overdose or hotspot in the target or a healthy tissue, for example), whereas a stricter evaluation criteria may reject dose delivery even though the detected dose discrepancies do not affect the patient.
Further, in established dose verification methods, if the measured radiation is different from the expected radiation, the treatment is stopped. However, if the radiation beam is tangential to the patient, a small change in the patient outline can make a significant difference in the measured dose while the actual dose in the patient is not affected significantly. In fact, in some instances, the radiation field is made intentionally larger than the target. In arc therapy treatments, for example, tangential fields are more likely to occur because all radiation directions in the plane are used. Thus, when all of the points irradiated by the beam are used in real-time evaluation of the treatment, the established dose evaluation methods may falsely detect a dose error and trigger stopping of the radiation treatment.